On combines and forage harvesters, as well as on other agricultural vehicles, brakes are used to turn the vehicle within a smaller turning circle than would be achievable by the use of the steering wheels. The brakes are also the preferred method of steering in difficult field conditions.
In order to achieve this, two separate braking circuits are provided which have separate brake pedals for braking the left and right sides of the vehicle. The brakes are designed to be very powerful so that steering using the brakes can be achieved with minimal effort.
Furthermore, in combine and forage harvesters the hydrostatic drive system is often used for braking. During field operation, the hydrostatic drive system serves as the primary means for stopping the vehicle while the friction brakes acting on the wheels are used primarily for steering.
Of course, the same braking systems must be capable of being used when the vehicle is being driven on roads. Under such driving conditions, the two brake pedals are physically connected to one another, so that they cannot be depressed separately, and symmetrical braking is achieved by hydraulically interconnecting the two braking systems so that the same braking pressure is applied to the slave cylinders on both sides of the vehicle.
As a result, for driving on normal roads, more braking capacity is available to the driver than is needed and in some countries there is a legal requirement for simultaneous braking using the hydrostatic drive system which increases the maximum braking force still further.
The availability of an excessively high braking force presents a particular problem in the case of harvesters that are being driven on a road in that they risk toppling forwards. This problem is aggravated by the fact that, when driven on a road, the header of a harvester, that is to say the attachment on the front of the vehicle which is operable to cut and collect the crop, has to be raised and therefore changes the position of the centre of gravity of the whole vehicle.